title: Mammographic density in birth cohorts of Danish women: a longitudinal study.
abstract: BACKGROUND
  Breast cancer is the leading malignant disease among western women with incidence increasing over time. High mammographic density is a well-established risk factor for breast cancer. We explored trends in mammographic density across birth cohorts to gain further insight into possible time trends in women's mammographic density that might explain the historical increase in breast cancer incidence.
  
  
  METHODS
  Data derived from two mammography screening programs in Denmark from 1991 to 2001, including on average 41,091 women from Copenhagen and 52,938 women from Funen aged 50-69. Mammographic density was assessed qualitatively (fatty or mixed/dense) by senior screening radiologists. The proportion of women with mixed/dense mammographic density was calculated by age at screening, screening period, and birth cohort. The Generalized Estimating Equations were used to calculate odds ratios and 95% confidence intervals. All statistical tests were two-sided.
  
  
  RESULTS
  The proportion of women with mixed/dense mammographic density increased from 45% among women born in the 1920s to 75-80% among women born in the 1940s. In Copenhagen, the age-adjusted odds ratio (95% CI) of mixed/dense mammographic density in women born in 1941-42 was 2.48 (2.22-2.76) compared with women born in 1921-22. In Funen, the age-adjusted odds ratio of mixed/dense mammographic density in women born in 1946-47 was 5.89 (5.32-6.51) compared with women born in 1924-25. Hormone use had a greater impact on mammographic density in birth cohorts of the 1920s compared with those of the 1940s.
  
  
  CONCLUSIONS
  We found suggestive evidence of a birth cohort pattern in mammographic density and an attenuated impact of hormone use in younger compared with older birth cohorts suggesting that postmenopausal mammographic density could be linked to changing exposures accumulated over time in women's lives.
authors.affiliation: Department of Public Health, University of Copenhagen, Copenhagen, Denmark. sohe@sund.ku.dk.
authors.forename: Sophie Sell
authors.initials: SS
authors.lastname: Hellmann
authors.forename: Elsebeth
authors.initials: E
authors.lastname: Lynge
authors.forename: Walter
authors.initials: W
authors.lastname: Schwartz
authors.forename: Ilse
authors.initials: I
authors.lastname: Vejborg
authors.forename: Sisse Helle
authors.initials: SH
authors.lastname: Njor
mesh_terms: D000367:Age Factors; D000368:Aged; D001940:Breast / Q000473:pathology*; D000071060:Breast Density; D001943:Breast Neoplasms / Q000000981:diagnostic imaging*; D003718:Denmark; D055088:Early Detection of Cancer; D004967:Estrogens / Q000008:administration & dosage; D005260:Female; D006801:Humans; D008137:Longitudinal Studies; D042361:Mammary Glands, Human / Q000002:abnormalities* / Q000187:drug effects; D008327:Mammography; D008403:Mass Screening; D008875:Middle Aged; D016017:Odds Ratio; D011372:Progestins / Q000008:administration & dosage; D012307:Risk Factors
chemical_list: D004967:Estrogens; D011372:Progestins